1. Field of the Invention
This invention relates to flat plate heat exchangers, and more particularly to light weight flat plate solar collectors.
2. Description of the Prior Art
There are three basic forms of heat transfer: conduction; convection; and radiation. In most flat plate heat exchangers, heat is transferred to the fluid by convection. Convection is a function of moving fluid properties. There are two types of convection: natural convection; and forced convection. Natural convection is caused by internal buoyancy forces and forced convection is caused by external forces, such as a pressure difference caused by a pump or fan. This invention, like most flat plate heat exchangers, relates to the transfer of heat to an internally contained flowing heat transfer fluid using forced convection. The standard equation for forced convection is EQU Q(BTU/hr)=H(BTU/hr ft.sup.2 .degree. F.).times.A(ft.sup.2).times..DELTA.T(.degree.F.)
where H is the film coefficient and A is the heat transfer surface area. This invention describes ways to increase "H" and "A" in flat plate heat exchangers and thus increase the amount of heat Q which can be transferred for a given temperature difference .DELTA.T between the fluid and the wetted surface. The film coefficient is a function of the velocity distribution of the fluid adjacent to the heat transfer surfaces. Euler's equations of motion for three dimensional incompressible fluid flow determine the flow distribution. EQU X-1/.rho. .differential.p/.differential.x=u .differential.u/ox+v .differential.u/.differential.y+w .differential.u/.differential.z+.differential.u/.differential.t Eq. 1 EQU Y-1/.rho. .differential.p/.differential.y=u .differential.v/.differential.x+v .differential.v/.differential.y+w .differential.v/.differential.z+.differential.v/.differential.t Eq. 2 EQU Z-1/.rho. .differential.p/.differential.z=u .differential.w/.differential.x+v .differential.w/.differential.y+w .differential.w/.differential.z+.differential.w/.differential.t Eq. 3
where x, y, z are the components of the extraneous body forces and u, v, and w are the velocity components in the x, y, and z directions, respectively.
For one dimensional flow, such as in a pipe, Equation 1 describes the fluid velocity as a function of x, the length along the pipe.
For horizontal pipe flow, Equation 1 simplifies to the Bernoulli Equation for steady-state flow between points 1 and 2. EQU P.sub.1 +.rho. u.sub.1.sup.2 /2g=P.sub.2 +.rho. u.sub.2.sup.2 /2g+.DELTA.P Eq. 4
For an efficient heat exchanger, it is desirable to keep the pressure loss .DELTA.P low. This invention does this by streamlining flow passages and guiding the flow.
For two dimensional flow such as occurs between two flat plates, Equations 1 and 2 apply. It is important to this invention to differentiate between one-dimensional fluid flow and two-dimensional fluid flow. They have different equations, different heat transfer coefficients, and different pressure losses. Fluid flow is essentially one-dimensional if its flow distribution, heat transfer coefficients, and pressure losses can be essentially defined by one-dimensional equations (e.g. Eq. 4) or one-dimensional experiments (e.g., pipe networks). Fluid flow is essentially two-dimensional if its flow distribution, heat transfer coefficients, and pressure losses can be essentially defined by two-dimensional equations, (for example, Eq. 1 and Eq. 2) or two-dimensional experiments (e.g. water tables). Fluid flow is essentially three-dimensional if three-dimensional equations (for example Eq. 1, Eq. 2 and Eq. 3) are required to essentially describes its fluid flow and heat transfer characteristics.
Because two-dimensional flow has an extra degree of freedom over one-dimensional flow, two-dimensional heat exchangers can be designed to have higher efficiencies than one-dimensional heat exchangers. Accordingly, three-dimensional heat exchangers such as finned tube radiators often have the highest efficiencies.
Most flat plate heat exchangers utilize one-dimensional fluid flow passages such as tubes fastened on one flat plate or channels formed between two essentially flat plates. One group of patents to which this invention is related uses internal baffles to form essentially one-dimensional flow channels in a rectangular serpentine pattern (Deminet U.S. Pat. Nos. 3,981,294; Tomchak 4,085,728; and Hobbs 4,048,981). There is a small group of flat plate heat exchangers which show essentially two-dimensional fluid flow passages utilizing parallel baffle means or manifolds to split the flow into two or more channels, most of which are essentially one-dimensional (Bausal U.S. Pat. Nos. 4,117,831; DiPerl 4,122,828; and Skrivseth 4,099,513). This invention utilizes various versions of streamlining and turbulent heat transfer augmentation found in three-dimensional applications (e.g. Lage U.S. Pat. No. 1,356,114) but not found in existing flat plate heat exchangers.